The invention concerns a distance measuring device, in particular for measuring the fill level of industrial tanks. The device is made of electronic transmitting- and receiving elements for short wave electromagnetic waves, such as microwaves, and a cylindrically-shaped waveguide which passes through a separating wall. Inside thereof is a waveguide window which separates the two spaces and is made of a material which is transmissive with respect to electromagnetic waves, such as quartz glass. The waveguide and the waveguide window have conically formed parts to provide axial support for them.
Also known, besides radar instruments for measuring distance, are level measuring devices operating with microwaves, which transmit signals in the microwave range which are reflected by the surface of the medium located in the container and received by the receiver. The distance between the level measuring device and the surface of the medium is determined from the signals with an electronic circuit which may include microprocessors and electronic calculators. To use such microwave level measuring devices with containers, especially industrial tanks, which are subject to high or low operating temperatures and low or high pressures, in particular when they contain explosive and/or corrosive and/or toxic materials, it is necessary to separate the interior of the container from the electronic transmitter- and receiver elements. This is commonly accomplished by arranging inside the waveguide which protrudes through the container ceiling a cylindrically-shaped waveguide window made of quartz glass or the like and which has a low dielectric dissipation factor favorable to the transmittance of microwaves. Thus, the waveguide window constitutes the separator which keeps the electronic elements apart from the content of the tank. When there is a pressure acting from the side of the housing or when there is low pressure on the side of the container, the danger exists that the waveguide window will be pushed out of the waveguide. To prevent this, it is known from U.S. Pat. No. 3,001,160 to provide the waveguide and the waveguide window with conically designed sections. The conical surfaces are in snug contact with each other. In this system the waveguide supports the waveguide window against axial forces, but it has the disadvantage that large temperature fluctuations subject the glass to considerable stress, so that the glass can break or crack. This results not only from the different thermal expansion coefficients of the glass and the waveguide tube, but also the differences in heat dissipation between the two.